Not applicable.
Not applicable.
The production of liquefied natural gas (LNG) is achieved by cooling and condensing a feed gas stream against multiple refrigerant streams provided by a recirculating refrigeration system. Cooling of the natural gas feed is accomplished by various cooling process cycles such as the well-known cascade cycle in which refrigeration is provided by three different refrigerant loops. One such cascade cycle uses methane, ethylene and propane cycles in sequence to produce refrigeration at three different temperature levels. Another well-known refrigeration cycle uses a propane precooled, mixed refrigerant cycle in which a multicomponent refrigerant mixture generates refrigeration over a selected temperature range. The mixed refrigerant can contain hydrocarbons such as methane, ethane, propane, and other light hydrocarbons, and also may contain nitrogen. Versions of this efficient refrigeration system are used in many operating LNG plants around the world.
Single or double mixed refrigerant cycles, with or without propane precooling, have been used for natural gas liquefaction. Single mixed refrigerant cycles have vaporized the mixed refrigerant either at one or at two different pressure levels to provide refrigeration over the required temperature range.
U.S. Pat. No. 4,251,247 discloses single mixed refrigerant systems in which the refrigerant vaporizes at two pressures. The compressed single mixed refrigerant stream either after compressor interstage cooling and/or after the final compressor stage cooling to near ambient temperature provides a liquid fraction and a vapor fraction. The refrigeration derived from the vapor fraction is used to provide some or all of the cooling of the natural gas from ambient temperature down to near xe2x88x9255xc2x0 C. The refrigeration from the liquid fraction is used for the cooling of the vapor fraction prior to recovery of the refrigeration from the cooled vapor fraction. In FIG. 4 of this patent, natural gas is first cooled from ambient temperature to an intermediate temperature by refrigeration derived from a combined stream which is derived by combining all of the liquid fraction with a portion of the vapor fraction. In FIG. 5 of this patent, natural gas from ambient temperature is cooled down to 20xc2x0 C. using refrigeration from a portion of the liquid fraction and is processed in an adsorption unit (dehydrating unit) for water removal. In order to avoid the formation of methane hydrates, natural gas is not cooled to temperatures much below 20xc2x0 C. prior to the adsorption unit. In order to cool natural gas from 37xc2x0 C. to 20xc2x0 C., a portion of the liquid refrigerant fraction is partially vaporized by heat exchange with the natural gas and is returned to a separator located at an interstage of the compressor. However, natural gas exiting the adsorption unit is cooled from 20xc2x0 C. to xe2x88x9254xc2x0 C. using refrigeration derived from the vapor fraction of the single mixed refrigerant stream.
A single mixed refrigerant system in which the refrigerant boils at two pressures is described in U.S. Pat. No. 3,747,359. Low pressure mixed refrigerant is compressed warm; that is, it is introduced into the compressor after heat exchange with warm natural gas feed and high pressure mixed refrigerant feeds. Intermediate pressure mixed refrigerant is obtained after cooling below ambient temperature rather than after ambient cooling, and no separation of mixed refrigerant occurs at ambient temperature.
U.S. Pat. No. 4,325,231 discloses a single mixed refrigerant system in which the refrigerant vaporizes at two pressures. The high pressure liquid condensed after ambient cooling is subcooled and vaporized at low pressure, while the high pressure vapor remaining after ambient cooling is further cooled yielding a second liquid and a second vapor stream. The second vapor stream is liquefied, subcooled and vaporized at low pressure, while the second liquid stream is subcooled and vaporized at low and intermediate pressures. Ambient temperature high pressure liquid and high pressure vapor streams are cooled in separate parallel heat exchangers. All vaporized mixed refrigerant streams are warmed to near ambient temperature prior to compression.
U.S. Pat. No. 5,657,643 describes a single mixed refrigerant system in which the refrigerant boils at one pressure. The compression of mixed refrigerant occurs in two stages and yields a liquid condensate after the intercooler which is pumped and mixed with the discharge of the final compression stage. Cooling of the feed and mixed refrigerant occur in a single multi-stream heat exchanger.
Improved efficiency of gas liquefaction processes is highly desirable and is the prime objective of new cycles being developed in the gas liquefaction art. The objectives of the present invention, as described below and as defined by the claims which follow, comprise improvements to liquefaction processes which use a single mixed refrigerant. The improvements include the compression of vaporized refrigerant at reduced compressor inlet temperatures and the generation of interstage liquid refrigerant streams at ambient temperature which can be used beneficially in the refrigeration cycle.
The invention relates to a method for gas liquefaction which comprises:
(a) cooling an essentially water-free feed gas by indirect heat exchange with one or more vaporizing liquid mixed refrigerant streams in a first cooling zone, wherein at least one of the liquid mixed refrigerant streams in the first cooling zone is vaporized at a first pressure level, and withdrawing from the first cooling zone an intermediate cooled feed gas and a first vaporized mixed refrigerant;
(b) further cooling the intermediate cooled feed gas by indirect heat exchange with one or more vaporizing liquid mixed refrigerant streams in a second cooling zone, wherein at least one of the liquid mixed refrigerant streams in the second cooling zone is vaporized at a second pressure level, and withdrawing from the second cooling zone a liquefied gas and a second vaporized mixed refrigerant; and
(c) compressing and cooling the first vaporized mixed refrigerant and the second vaporized mixed refrigerant to yield one or more liquid mixed refrigerant streams, wherein the cooling is ambient cooling effected by heat transfer to an ambient heat sink.
The one or more vaporizing liquid mixed refrigerant streams utilized to cool the feed gas in the first cooling zone of (a) may be derived solely from the one or more liquid mixed refrigerant streams of (c).
The vaporizing liquid mixed refrigerant streams in the first and second cooling zones may be provided in a recirculating refrigeration process which includes the steps of:
(1) compressing the second vaporized mixed refrigerant to a first pressure level to yield a pressurized second mixed refrigerant;
(2) combining the pressurized second mixed refrigerant with the first vaporized mixed refrigerant and compressing the resulting combined refrigerant stream to yield a compressed mixed refrigerant stream;
(3) cooling and partially condensing the compressed mixed refrigerant stream by ambient cooling to yield a mixed refrigerant vapor and a mixed refrigerant liquid;
(4) subcooling and reducing the pressure of the mixed refrigerant liquid to provide a vaporizing liquid mixed refrigerant stream in the first cooling zone at the first pressure level; and
(5) cooling, at least partially condensing, and reducing the pressure of the mixed refrigerant vapor to provide a vaporizing liquid mixed refrigerant which is vaporized in the second cooling zone at a second pressure level;
The compression of the combined regrigerant stream in (2) may be effected in multiple stages of compression, and an interstage vapor refrigerant stream may be cooled and partially condensed by ambient cooling to yield an additional mixed refrigerant liquid.
The essentially water-free feed gas may be provided by removing water from a natural gas feed stream. Optionally, the additional mixed refrigerant liquid can be pressurized by pumping and the resulting pressurized liquid combined with the compressed mixed refrigerant stream. If desired, the additional mixed refrigerant liquid can be subcooled and reduced in pressure to provide another vaporizing liquid mixed refrigerant stream in the first cooling zone.
A portion of the refrigeration for cooling and partially condensing the mixed refrigerant vapor in (e) above can be provided by the vaporizing liquid mixed refrigerant stream in the first cooling zone. Another portion of the refrigeration for cooling and partially condensing the mixed refrigerant vapor in (e) can be provided at least in part by the vaporizing liquid mixed refrigerant stream in the second cooling zone. At least a portion of the refrigeration for subcooling of the mixed refrigerant liquid in (d) can be provided by the vaporizing liquid mixed refrigerant stream in the first cooling zone. The refrigeration for subcooling the additional mixed refrigerant liquid can be provided at least in part by the vaporizing liquid mixed refrigerant stream in the first cooling zone.
In an optional embodiment, the mixed refrigerant vapor can be cooled, partially condensed, and separated into a second mixed refrigerant vapor and a second mixed refrigerant liquid. The second mixed refrigerant liquid can be subcooled and reduced in pressure to provide a vaporizing liquid mixed refrigerant stream in the second cooling zone. The refrigeration for subcooling the second mixed refrigerant liquid can be provided in part by the vaporizing liquid mixed refrigerant stream which is vaporized in the second cooling zone. The second mixed refrigerant vapor can be cooled, at least partially condensed, and reduced in pressure to provide another vaporizing liquid mixed refrigerant stream in the second cooling zone.
The refrigeration for cooling the second mixed refrigerant vapor can be provided at least in part by the vaporizing liquid mixed refrigerant stream in the second cooling zone. A portion of the mixed refrigerant liquid after subcooling in (d) can be combined with the second mixed refrigerant liquid, and the resulting combined stream can be subcooled, reduced in pressure, and vaporized at the second pressure level in the second cooling zone.
The intermediate cooled feed gas preferably is at a temperature below about 10xc2x0 C.